1. 1. Field of the Invention
The present invention relates to a semiconductor light emitting device having a semiconductor superlattice structure including a quantum well layer and a barrier layer, such as a light emitting diode (hereinafter, referred to as an "LED") and a semiconductor laser.
2. Description of the Related Art
Conventionally, a quantum well structure and a superlattice structure in a semiconductor light emitting device such as an LED and a semiconductor laser are widely used as a light emitting layer in order to remarkably improve light emitting characteristics such as emission efficiency and temperature characteristic and shorten the wavelength of the emitted light.
FIG. 12 is a view illustrating a band line-up (band alignment) of a conventional LED using a quantum well structure for an active layer. The conventional LED shown in FIG. 12 uses a heterojunction of Al.sub..chi. Ga.sub.1. .sub..chi. As/GaAs-type materials. The band line-up shown in FIG. 12, which is obtained using a heterojunction of GaAs/AlGaAs-type materials, is referred to as a "type I" or "straddling type". An electron affinity (.chi.a and .chi.b), which is an energy required to bring electrons from the vacuum level to the bottom of the conduction band, and a band energy gap (Ea and Eb) have the relationship of: EQU .chi.a&gt;.chi.b; and EQU .chi.a+Ea&lt;.chi.b+Eb.
Generally in an LED using such materials, a cladding layer acting as a barrier layer has heterobarrier differences or band offsets of .DELTA.Ev and .DELTA.Ec with respect to electrons and holes as carriers which are injected to the active layer contributing to light emission. Due to such heterobarriers, the carriers can be effectively confined in the active layer. In the case of using a quantum well structure for an active layer as is shown in FIG. 12, the carriers can be effectively injected only into a quantum well acting as an active layer. Such a structure significantly improves the characteristics of an LED and a semiconductor laser.
The above-described structure is effective for a compound semiconductor containing GaAs/AlGaAs-type materials which are most generally used. On the other hand, in a device which uses a heterojunction of GaP/AlGaP-type materials in order to increase the band gap so as to realize light emission of shorter wavelengths, a band line-up which is referred to as "type II" or "staggered type" is generated. In such a band line-up, the electron affinity (.chi.a and .chi.b) and the band energy gap (Ea and Eb) have the relationship of: EQU .chi.a&lt;.chi.b; and EQU .chi.a+Ea&lt;.chi.b+Eb.
In this specification, a layer having a smaller electron affinity .chi.a in the type II band alignment is referred to as a "well layer", and a layer having a larger electron affinity .chi.b is referred to as a "barrier layer".
In a device using the GaP/AlGaP-type materials, a heterobarrier formed between a cladding layer as a barrier layer and an active layer as a well layer is higher in the cladding layer with respect to a valence band and is higher in the active layer with respect to the conduction band. Therefore, in the type II heterojunction superlattice structure, as opposed to the type I heterojunction superlattice structure, holes are confined in the well layer acting as an active layer, and electrons are confined in the barrier layer acting as a cladding layer, but not in the well layer. As a result, the emission efficiency cannot be easily improved even by a heterojunction superlattice structure.
An example of a laser oscillator including a type II superlattice structure using InP/AlInAs-type semiconductor materials is described in Appl. Phys. Lett. 60 (25), pp. 3087-3089. However, it is described that oscillation is difficult even if the laser chip is cooled down to a liquid nitrogen temperature. Accordingly, in the case when the GaP/AlGaP-type materials are used, improvement in emission efficiency using a quantum well structure is impossible, and confinement of carriers by use of double heterojunction is difficult. Under these circumstances, only LEDs using homo-junction are conventionally used.
FIG. 14 is a diagram of a band line-up of a conventional LED having a homo-junction structure of GaP As is shown in FIG. 14, in the case when Ga.sub.1-x Al.sub.x P-type (x=0 to 1) semiconductor materials are used, the band gap energy is 2.25 to 2.45 eV. Such a range or energy corresponds to emission of green light, but the luminescence intensity is low due to indirect transition and thus is not practical. When such materials are used, the emission efficiency is improved by intentionally forming an emission center by doping. In detail, a V-group atom contained in such semiconductor materials is replaced with another V-group atom so as to form an isoelectronic trap in the semiconductor materials. In the case where nitrogen is selected as the isoelectronic trap, an emission level is formed at a position which is lower than the conduction band by .DELTA.Ei=50 meV. The wavelength of the light emitted by such a structure is obtained by: EQU Ei=Ea-.DELTA.Ei (1)
where Ea is the transition energy. By substituting Ea=2.25 eV and .DELTA.Ei=50 meV to equation (1), Ei=2.20 eV is found. From this, the wavelength of the emitted light is 565 nm.
FIG. 15 is a cross sectional view of a GaP LED 6 using nitrogen as an isoelectronic trap. The GaP LED 6 includes an n-type GaP substrate 1, an n-type GaP:N layer 2, and an p-type GaP:N layer 3 stacked in this order. Such a three-layer stack is interposed between electrodes 4 and 5. The GaP LED 6 having such a structure emits green light. The GaP LED 6 emits light having a luminance of 600 mcd when employed in a standard product (5 mm.PHI. mold lamp, 20 mA).
Japanese Laid-Open Patent Publication No. 63-128776 discloses a superlattice light emitting device formed of a GaP doped with nitrogen as an isoelectronic trap and AlGaP used for a barrier layer in order to improve the emission efficiency and shorten the wavelength of the emitted light. Since the type I double heterojunction structure cannot be formed for the above-described reason, the emission efficiency cannot be improved or the wavelength cannot be shortened easily beyond the level which is realized by homo-junction.